[OSM-dev] Scale of downloaded images seems to vary.
Bjoern Hassler
bjohas+mw at gmail.com
Thu Jan 11 10:38:57 UTC 2018
Hi Tom, hi Darafei, hi Yves, dear friends,
thank you for your messages!
(1) DPI on exported images
Actually isn't the real problem here just needing to know what DPI is being
> being assumed by mapnik when rendering?
> IIRC it's 96dpi?
As you say (and also in the link Yves posted) it's about 90.7dpi, standard
pixel size of 0.28 millimeters as defined by the OGC (Open Geospatial
Consortium) SLD (Styled Layer Descriptor). However, the images downloaded
are claiming 72dpi. I need to check whether they don't have dpi metadata
(and therefore my OS assumes 72) or whether they are tagged incorrectly.
Either way, that sounds like a discrepancy though, right?
(2) "project the bounding box from EPSG:4326 lat/lon to spherical mercator
which gives us coordinates on a projected sheet measuring roughly 40075016m
on each edge (2 x PI x assumed earth radius)."
That's very helpful. What is the function (of latitude) that maps the sheet
to the smaller scale map? I'd assumed cos(lat), but it seems to be more
complicated. I can dig around, but if you happen to know it, then don't
need to dig around. The code has this:
scale = float(form.getvalue("scale"))
width = int(bbox.width() / scale / 0.00028)
height = int(bbox.height() / scale / 0.00028)
where I assume form.getvalue fetches the "mapnik_" form. So, equivalently,
it would be about the variation of the bbox.width() with lat, in
the spherical mercator projection. Any thoughts on this?
(3) Filing a bug: Clearly there's nothing wrong with the rendering (bar
issue 1 above?). However, one might say that the 'scale' label on the box
is misleading to a casual user, as users would assume geographical scale,
rather than mapnik_scale. Would it be helpful to users to have a few words
underneath to the scale box, saying "Note that this is not geographical
scale." and link to page that explained how to convert?
Many thanks,
Bjoern
On 11 January 2018 at 09:54, Tom Hughes <tom at compton.nu> wrote:
> Looks like we are using 90dpi actually. The key code is here:
>
> https://github.com/openstreetmap/chef/blob/master/cookbooks/
> tile/templates/default/export.erb#L121
>
> We project the bounding box from EPSG:4326 lat/lon to spherical mercator
> which gives us coordinates on a projected sheet measuring roughly 40075016m
> on each edge (2 x PI x assumed earth radius).
>
> We then divide by the scale factor and then convert metres to pixels by
> dividing by 0.00028 which comes from:
>
> 1 / 39.701 / 90
>
> Where we divide by 39.701 to convert from metres to inches and then by a
> further 90 to convert to pixels at 90 dpi.
>
> Tom
>
>
> On 11/01/18 08:52, Tom Hughes wrote:
>
>> Actually isn't the real problem here just needing to know what DPI is
>> being being assumed by mapnik when rendering?
>>
>> IIRC it's 96dpi?
>>
>> Tom
>>
>> On 11/01/18 08:50, Tom Hughes wrote:
>>
>>> Please don't - it has nothing to do with the web site code.
>>>
>>> Please go and read about projections instead.
>>>
>>> Tom
>>>
>>> On 11/01/18 08:21, Darafei "Komяpa" Praliaskouski wrote:
>>>
>>>> Hi,
>>>>
>>>> As long as there is nobody on the list can make sense of the values for
>>>> almost two weeks, I'd say it should be considered a bug and filed towards
>>>> https://github.com/openstreetmap/openstreetmap-website/issues
>>>>
>>>> чт, 11 янв. 2018 г. в 2:05, Bjoern Hassler <bjohas+mw at gmail.com
>>>> <mailto:bjohas%2Bmw at gmail.com>>:
>>>>
>>>> Dear friends,
>>>>
>>>> I was just wondering whether anybody else had any thoughts on this?
>>>> Any tips on making sense of the mapnik_scale would be greatly
>>>> appreciated!
>>>>
>>>> Bjoern
>>>>
>>>> On 3 January 2018 at 17:21, Bjoern Hassler <bjohas+mw at gmail.com
>>>> <mailto:bjohas+mw at gmail.com>> wrote:
>>>>
>>>> Hi Bryan, hi Darafei,
>>>>
>>>> That's helpful, thanks. So we know that the calculation from the
>>>> bbox is correct.
>>>>
>>>> However, I guess we don't know about how pixels translate to
>>>> real-word dims? (Or, equicvalenly, how pixels relate to the
>>>> lat-lon extent.)
>>>>
>>>> Thanks!
>>>> Bjoern
>>>>
>>>> On 2 January 2018 at 14:46, Bryan Housel <bryan at 7thposition.com
>>>> <mailto:bryan at 7thposition.com>> wrote:
>>>>
>>>> Bjoern, maybe the geo functions used in iD might be a
>>>> helpful reference:
>>>> https://github.com/openstreetmap/iD/blob/master/modules/geo/geo.js
>>>>
>>>> The numbers I got from comparing the bbox sizes are pretty
>>>> close to your numbers.
>>>>
>>>>
>>>> bbox1 = [[24.123255,49.250507], [24.234286,49.367924]]
>>>>
>>>> dLat1 = bbox1[1][0] - bbox1[0][0]
>>>> > 0.11103100000000055
>>>> iD.geoLatToMeters(dLat1)
>>>> > 12359.91438226802
>>>> dLon1 = bbox1[1][1] - bbox1[0][1]
>>>> > 0.11741700000000321
>>>> iD.geoLonToMeters(dLon1, (bbox1[1][0] + bbox1[0][0])/2)
>>>> > 11884.145336433623
>>>>
>>>> (image1 is 11.884 km x 12.359 km)
>>>>
>>>>
>>>> bbox2 = [[48.632228,-101.369133], [48.691074,-101.251717]]
>>>>
>>>> dLat2 = bbox2[1][0] - bbox2[0][0]
>>>> > 0.05884600000000262
>>>> iD.geoLatToMeters(dLat2)
>>>> > 6550.706755221268
>>>> dLon2 = bbox2[1][1] - bbox2[0][1]
>>>> > 0.11741600000000574
>>>> iD.geoLonToMeters(dLon2, (bbox2[1][0] + bbox2[0][0])/2)
>>>> > 8604.30156213755
>>>>
>>>> (image2 is 8.604 km x 6.550 km)
>>>>
>>>>
>>>> Bryan
>>>>
>>>>
>>>>
>>>> On Jan 1, 2018, at 6:56 AM, Bjoern Hassler
>>>>> <bjohas+mw at gmail.com <mailto:bjohas+mw at gmail.com>> wrote:
>>>>>
>>>>> Hi Darafei, dear all,
>>>>>
>>>>> Thanks, but I still cannot get this to work.
>>>>>
>>>>> I've now calculated real_scale = mapnik_scale / cos(lat),
>>>>> and used the real_scale, to calculate:
>>>>>
>>>>> pixels * (72/2.54 pixels/cm) * real_scale = real_world_dim
>>>>>
>>>>> However, there's still a latitude-dependent discrepancy
>>>>> (see below). I could try to fit that to latitude, to see
>>>>> what the formula is, but I'm hoping somebody has the
>>>>> answer (or can let me know what I got wrong!)
>>>>>
>>>>> Happy new year!
>>>>> Bjoern
>>>>>
>>>>> *Example 1:*
>>>>> http://www.openstreetmap.org/#map=13/24.1788/49.3092
>>>>> bbox = [24.123255,49.250507; 24.234286,49.367924]
>>>>> bbox size in degrees (lon, lat) = 0.117416, 0.111031
>>>>> *Pixels ('Image ... at'):* 1945 x 2016;
>>>>> *mapnik_scale* 1 : 24000; *real scale *1 : 26308
>>>>> Image dim (1 : 26308, 72dpi): 686 mm x 711 mm
>>>>> Real world dim (1:1, from pixels): *18.051 km x 18.71 km*
>>>>> Real world dim (1:1, latlon): *11.911 km x 12.346 km*
>>>>> Ratio (dim pixels/ dim latlon): 1.516 ; 1.515
>>>>>
>>>>> *Example 2:*
>>>>> http://www.openstreetmap.org/#map=13/48.6617/-101.3104
>>>>> bbox = [48.632228,-101.369133; 48.691074,-101.251717]
>>>>> bbox size in degrees (lon, lat) = 0.117416, 0.058846
>>>>> *Pixels ('Image ... at'):* 1945 x 1476;
>>>>> *mapnik_scale* 1 : 24000; *real scale *1 : 36336
>>>>> Image dim (1 : 36336, 72dpi): 686 mm x 521 mm
>>>>> Real world dim (1:1, from pixels): *24.932 km x 18.92 km*
>>>>> Real world dim (1:1, latlon):*8.624 km x 6.543 km*
>>>>> Ratio (dim pixels/ dim latlon): 2.891 ; 2.891
>>>>>
>>>>>
>>>>>
>>>>> On 31 December 2017 at 18:59, Darafei "Komяpa"
>>>>> Praliaskouski <me at komzpa.net <mailto:me at komzpa.net>>
>>>>> wrote:
>>>>>
>>>>> Images are in Spherical Mercator EPSG:3857 projection,
>>>>> so linear scale is off by cos(lat).
>>>>>
>>>>>
>>>>> On Sun, Dec 31, 2017, 20:07 Bjoern Hassler
>>>>> <bjohas+mw at gmail.com <mailto:bjohas%2Bmw at gmail.com>>
>>>>> wrote:
>>>>>
>>>>> Dear friends,
>>>>>
>>>>> I'm trying to make sense of the scales for map
>>>>> images downloaded from OSM. For the download, you
>>>>> can choose the scale, and I had assumed that I
>>>>> could use this to convert to an actual map scale.
>>>>>
>>>>> The downloaded png/jpg etc seem to be at 72dpi. I
>>>>> had assumed I could just convert pixels at 72dpi
>>>>> to actual dimensions (using the scale).
>>>>>
>>>>> However - as far as I can tell - this doesn't
>>>>> work. Maybe I've made a mistake somewhere, but the
>>>>> dimensions calculated from
>>>>>
>>>>> * "feature in pixels" / (72/2.54 pixels/cm) *
>>>>> scale = "feature size" in cm
>>>>> * lat-lon (e.g. bounding box provided)
>>>>>
>>>>> Doesn't match. Moreover, the difference doesn't
>>>>> seem to be a constant offset or ratio, but
>>>>> possibly latitude dependent.
>>>>>
>>>>> Maybe the scale offered during download is not
>>>>> meant to be a geographic scale? Maybe I've
>>>>> misunderstood something?
>>>>> There are two worked examples below, that show the
>>>>> issue.
>>>>>
>>>>> Any thoughts?
>>>>> Bjoern
>>>>>
>>>>> (and a Happy New Year!!)
>>>>>
>>>>>
>>>>> *Example 1:*
>>>>>
>>>>> I had a look for long straight roads ... (Trivia:
>>>>> http://www.dangerousroads.org/rankings23/3759-the-10-longest
>>>>> -straight-roads-in-the-world.html
>>>>> - "Located in the heart of Saudi Arabia, the
>>>>> Highway 10 is 120 miles (193km) stretch of
>>>>> straightness. This asphalted road links Haradh and
>>>>> Al Batha. It’s a straight road running right
>>>>> through the desert for 2 h 1 min.")
>>>>>
>>>>> - Open 'share',
>>>>> - set scale to 1:50000,
>>>>> - adjust view port so that "Image will show
>>>>> standard layer at 932x..."
>>>>> - Go here:
>>>>> http://www.openstreetmap.org/#
>>>>> map=13/24.1349/49.3083
>>>>>
>>>>> On the map, there's a road (East/West), with two
>>>>> turn-off: First, a power line at the Eastern edge
>>>>> (running North/South). In the west, there are two
>>>>> turn-off, the second (straight one) being 11.9 km
>>>>> from the power line (according to JOSM). In the
>>>>> image, you've got those right at the edges. From
>>>>> the bounding box (hidden fields), I calculate
>>>>> 11.62km. Given that the roads are just showing
>>>>> either side of the image, that's bang on.
>>>>>
>>>>> Now download PNG, which will have with 932. I am
>>>>> assuming I have a PNG (72dpi = 28.35 dots per cm),
>>>>> at scale 1:50,000. I calculate:
>>>>>
>>>>> 932 pixels / (72/2.54 pixels/cm) * 50000 = 16.4 km.
>>>>>
>>>>> So there's a difference between the dimensions
>>>>> calculated from the pixels and the distance
>>>>> calculated from lat/lon.
>>>>>
>>>>> *Full details for Example 1:*
>>>>>
>>>>> Z/L/L #13/24.1727/49.3090
>>>>> bbox = [24.119651808471247,49.249992370605476 ->
>>>>> 24.22567631717543,49.368095397949226]
>>>>> Pixel dim: 939 x 924;
>>>>> Natural image dim (72dpi): 331 mm x 326 mm, 1 :
>>>>> 50000
>>>>> Real world dim (from pixels): 16.563 km x 16.298
>>>>> km, 1 : 1
>>>>> Real world dim (latlon): 11.981 km x 11.789 km, 1
>>>>> : 1
>>>>> Ratio: 1.382438861530757 ; 1.3824751887352615
>>>>>
>>>>> *Example 2:*
>>>>>
>>>>> Another example from the above list:
>>>>>
>>>>> Z/L/L #13/48.6536/-101.3485
>>>>> bbox = [48.615207636211146,-101.44741058349611 ->
>>>>> 48.69198023486001,-101.24965667724611]
>>>>> Pixel dim: 1572 x 924;
>>>>> Natural image dim (72dpi): 555 mm x 326 mm, 1 :
>>>>> 50000
>>>>> Real world dim (from pixels): 27.728 km x 16.298
>>>>> km, 1 : 1
>>>>> Real world dim (latlon): 14.526 km x 8.537 km, 1 :
>>>>> 1
>>>>> Ratio: 1.908853091009225 ; 1.909101557924329
>>>>>
>>>>> The distance (along the highway) from the turnoffs
>>>>> to Undip / Lansford airstrips is 8.1km in JOSM. So
>>>>> the latlon calculation is correct. However, the
>>>>> dimension calculated from the pixels isn't.
>>>>> _______________________________________________
>>>>> dev mailing list
>>>>> dev at openstreetmap.org <mailto:
>>>>> dev at openstreetmap.org>
>>>>> https://lists.openstreetmap.org/listinfo/dev
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> dev mailing list
>>>>> dev at openstreetmap.org <mailto:dev at openstreetmap.org>
>>>>> https://lists.openstreetmap.org/listinfo/dev
>>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> _______________________________________________
>>>> dev mailing list
>>>> dev at openstreetmap.org
>>>> https://lists.openstreetmap.org/listinfo/dev
>>>>
>>>>
>>>
>>>
>>
>>
>
> --
> Tom Hughes (tom at compton.nu)
> http://compton.nu/
>
On 11 January 2018 at 09:54, Tom Hughes <tom at compton.nu> wrote:
> Looks like we are using 90dpi actually. The key code is here:
>
> https://github.com/openstreetmap/chef/blob/master/cookbooks/
> tile/templates/default/export.erb#L121
>
> We project the bounding box from EPSG:4326 lat/lon to spherical mercator
> which gives us coordinates on a projected sheet measuring roughly 40075016m
> on each edge (2 x PI x assumed earth radius).
>
> We then divide by the scale factor and then convert metres to pixels by
> dividing by 0.00028 which comes from:
>
> 1 / 39.701 / 90
>
> Where we divide by 39.701 to convert from metres to inches and then by a
> further 90 to convert to pixels at 90 dpi.
>
> Tom
>
>
> On 11/01/18 08:52, Tom Hughes wrote:
>
>> Actually isn't the real problem here just needing to know what DPI is
>> being being assumed by mapnik when rendering?
>>
>> IIRC it's 96dpi?
>>
>> Tom
>>
>> On 11/01/18 08:50, Tom Hughes wrote:
>>
>>> Please don't - it has nothing to do with the web site code.
>>>
>>> Please go and read about projections instead.
>>>
>>> Tom
>>>
>>> On 11/01/18 08:21, Darafei "Komяpa" Praliaskouski wrote:
>>>
>>>> Hi,
>>>>
>>>> As long as there is nobody on the list can make sense of the values for
>>>> almost two weeks, I'd say it should be considered a bug and filed towards
>>>> https://github.com/openstreetmap/openstreetmap-website/issues
>>>>
>>>> чт, 11 янв. 2018 г. в 2:05, Bjoern Hassler <bjohas+mw at gmail.com
>>>> <mailto:bjohas%2Bmw at gmail.com>>:
>>>>
>>>> Dear friends,
>>>>
>>>> I was just wondering whether anybody else had any thoughts on this?
>>>> Any tips on making sense of the mapnik_scale would be greatly
>>>> appreciated!
>>>>
>>>> Bjoern
>>>>
>>>> On 3 January 2018 at 17:21, Bjoern Hassler <bjohas+mw at gmail.com
>>>> <mailto:bjohas+mw at gmail.com>> wrote:
>>>>
>>>> Hi Bryan, hi Darafei,
>>>>
>>>> That's helpful, thanks. So we know that the calculation from the
>>>> bbox is correct.
>>>>
>>>> However, I guess we don't know about how pixels translate to
>>>> real-word dims? (Or, equicvalenly, how pixels relate to the
>>>> lat-lon extent.)
>>>>
>>>> Thanks!
>>>> Bjoern
>>>>
>>>> On 2 January 2018 at 14:46, Bryan Housel <bryan at 7thposition.com
>>>> <mailto:bryan at 7thposition.com>> wrote:
>>>>
>>>> Bjoern, maybe the geo functions used in iD might be a
>>>> helpful reference:
>>>> https://github.com/openstreetmap/iD/blob/master/modules/geo/geo.js
>>>>
>>>> The numbers I got from comparing the bbox sizes are pretty
>>>> close to your numbers.
>>>>
>>>>
>>>> bbox1 = [[24.123255,49.250507], [24.234286,49.367924]]
>>>>
>>>> dLat1 = bbox1[1][0] - bbox1[0][0]
>>>> > 0.11103100000000055
>>>> iD.geoLatToMeters(dLat1)
>>>> > 12359.91438226802
>>>> dLon1 = bbox1[1][1] - bbox1[0][1]
>>>> > 0.11741700000000321
>>>> iD.geoLonToMeters(dLon1, (bbox1[1][0] + bbox1[0][0])/2)
>>>> > 11884.145336433623
>>>>
>>>> (image1 is 11.884 km x 12.359 km)
>>>>
>>>>
>>>> bbox2 = [[48.632228,-101.369133], [48.691074,-101.251717]]
>>>>
>>>> dLat2 = bbox2[1][0] - bbox2[0][0]
>>>> > 0.05884600000000262
>>>> iD.geoLatToMeters(dLat2)
>>>> > 6550.706755221268
>>>> dLon2 = bbox2[1][1] - bbox2[0][1]
>>>> > 0.11741600000000574
>>>> iD.geoLonToMeters(dLon2, (bbox2[1][0] + bbox2[0][0])/2)
>>>> > 8604.30156213755
>>>>
>>>> (image2 is 8.604 km x 6.550 km)
>>>>
>>>>
>>>> Bryan
>>>>
>>>>
>>>>
>>>> On Jan 1, 2018, at 6:56 AM, Bjoern Hassler
>>>>> <bjohas+mw at gmail.com <mailto:bjohas+mw at gmail.com>> wrote:
>>>>>
>>>>> Hi Darafei, dear all,
>>>>>
>>>>> Thanks, but I still cannot get this to work.
>>>>>
>>>>> I've now calculated real_scale = mapnik_scale / cos(lat),
>>>>> and used the real_scale, to calculate:
>>>>>
>>>>> pixels * (72/2.54 pixels/cm) * real_scale = real_world_dim
>>>>>
>>>>> However, there's still a latitude-dependent discrepancy
>>>>> (see below). I could try to fit that to latitude, to see
>>>>> what the formula is, but I'm hoping somebody has the
>>>>> answer (or can let me know what I got wrong!)
>>>>>
>>>>> Happy new year!
>>>>> Bjoern
>>>>>
>>>>> *Example 1:*
>>>>> http://www.openstreetmap.org/#map=13/24.1788/49.3092
>>>>> bbox = [24.123255,49.250507; 24.234286,49.367924]
>>>>> bbox size in degrees (lon, lat) = 0.117416, 0.111031
>>>>> *Pixels ('Image ... at'):* 1945 x 2016;
>>>>> *mapnik_scale* 1 : 24000; *real scale *1 : 26308
>>>>> Image dim (1 : 26308, 72dpi): 686 mm x 711 mm
>>>>> Real world dim (1:1, from pixels): *18.051 km x 18.71 km*
>>>>> Real world dim (1:1, latlon): *11.911 km x 12.346 km*
>>>>> Ratio (dim pixels/ dim latlon): 1.516 ; 1.515
>>>>>
>>>>> *Example 2:*
>>>>> http://www.openstreetmap.org/#map=13/48.6617/-101.3104
>>>>> bbox = [48.632228,-101.369133; 48.691074,-101.251717]
>>>>> bbox size in degrees (lon, lat) = 0.117416, 0.058846
>>>>> *Pixels ('Image ... at'):* 1945 x 1476;
>>>>> *mapnik_scale* 1 : 24000; *real scale *1 : 36336
>>>>> Image dim (1 : 36336, 72dpi): 686 mm x 521 mm
>>>>> Real world dim (1:1, from pixels): *24.932 km x 18.92 km*
>>>>> Real world dim (1:1, latlon):*8.624 km x 6.543 km*
>>>>> Ratio (dim pixels/ dim latlon): 2.891 ; 2.891
>>>>>
>>>>>
>>>>>
>>>>> On 31 December 2017 at 18:59, Darafei "Komяpa"
>>>>> Praliaskouski <me at komzpa.net <mailto:me at komzpa.net>>
>>>>> wrote:
>>>>>
>>>>> Images are in Spherical Mercator EPSG:3857 projection,
>>>>> so linear scale is off by cos(lat).
>>>>>
>>>>>
>>>>> On Sun, Dec 31, 2017, 20:07 Bjoern Hassler
>>>>> <bjohas+mw at gmail.com <mailto:bjohas%2Bmw at gmail.com>>
>>>>> wrote:
>>>>>
>>>>> Dear friends,
>>>>>
>>>>> I'm trying to make sense of the scales for map
>>>>> images downloaded from OSM. For the download, you
>>>>> can choose the scale, and I had assumed that I
>>>>> could use this to convert to an actual map scale.
>>>>>
>>>>> The downloaded png/jpg etc seem to be at 72dpi. I
>>>>> had assumed I could just convert pixels at 72dpi
>>>>> to actual dimensions (using the scale).
>>>>>
>>>>> However - as far as I can tell - this doesn't
>>>>> work. Maybe I've made a mistake somewhere, but the
>>>>> dimensions calculated from
>>>>>
>>>>> * "feature in pixels" / (72/2.54 pixels/cm) *
>>>>> scale = "feature size" in cm
>>>>> * lat-lon (e.g. bounding box provided)
>>>>>
>>>>> Doesn't match. Moreover, the difference doesn't
>>>>> seem to be a constant offset or ratio, but
>>>>> possibly latitude dependent.
>>>>>
>>>>> Maybe the scale offered during download is not
>>>>> meant to be a geographic scale? Maybe I've
>>>>> misunderstood something?
>>>>> There are two worked examples below, that show the
>>>>> issue.
>>>>>
>>>>> Any thoughts?
>>>>> Bjoern
>>>>>
>>>>> (and a Happy New Year!!)
>>>>>
>>>>>
>>>>> *Example 1:*
>>>>>
>>>>> I had a look for long straight roads ... (Trivia:
>>>>> http://www.dangerousroads.org/rankings23/3759-the-10-longest
>>>>> -straight-roads-in-the-world.html
>>>>> - "Located in the heart of Saudi Arabia, the
>>>>> Highway 10 is 120 miles (193km) stretch of
>>>>> straightness. This asphalted road links Haradh and
>>>>> Al Batha. It’s a straight road running right
>>>>> through the desert for 2 h 1 min.")
>>>>>
>>>>> - Open 'share',
>>>>> - set scale to 1:50000,
>>>>> - adjust view port so that "Image will show
>>>>> standard layer at 932x..."
>>>>> - Go here:
>>>>> http://www.openstreetmap.org/#
>>>>> map=13/24.1349/49.3083
>>>>>
>>>>> On the map, there's a road (East/West), with two
>>>>> turn-off: First, a power line at the Eastern edge
>>>>> (running North/South). In the west, there are two
>>>>> turn-off, the second (straight one) being 11.9 km
>>>>> from the power line (according to JOSM). In the
>>>>> image, you've got those right at the edges. From
>>>>> the bounding box (hidden fields), I calculate
>>>>> 11.62km. Given that the roads are just showing
>>>>> either side of the image, that's bang on.
>>>>>
>>>>> Now download PNG, which will have with 932. I am
>>>>> assuming I have a PNG (72dpi = 28.35 dots per cm),
>>>>> at scale 1:50,000. I calculate:
>>>>>
>>>>> 932 pixels / (72/2.54 pixels/cm) * 50000 = 16.4 km.
>>>>>
>>>>> So there's a difference between the dimensions
>>>>> calculated from the pixels and the distance
>>>>> calculated from lat/lon.
>>>>>
>>>>> *Full details for Example 1:*
>>>>>
>>>>> Z/L/L #13/24.1727/49.3090
>>>>> bbox = [24.119651808471247,49.249992370605476 ->
>>>>> 24.22567631717543,49.368095397949226]
>>>>> Pixel dim: 939 x 924;
>>>>> Natural image dim (72dpi): 331 mm x 326 mm, 1 :
>>>>> 50000
>>>>> Real world dim (from pixels): 16.563 km x 16.298
>>>>> km, 1 : 1
>>>>> Real world dim (latlon): 11.981 km x 11.789 km, 1
>>>>> : 1
>>>>> Ratio: 1.382438861530757 ; 1.3824751887352615
>>>>>
>>>>> *Example 2:*
>>>>>
>>>>> Another example from the above list:
>>>>>
>>>>> Z/L/L #13/48.6536/-101.3485
>>>>> bbox = [48.615207636211146,-101.44741058349611 ->
>>>>> 48.69198023486001,-101.24965667724611]
>>>>> Pixel dim: 1572 x 924;
>>>>> Natural image dim (72dpi): 555 mm x 326 mm, 1 :
>>>>> 50000
>>>>> Real world dim (from pixels): 27.728 km x 16.298
>>>>> km, 1 : 1
>>>>> Real world dim (latlon): 14.526 km x 8.537 km, 1 :
>>>>> 1
>>>>> Ratio: 1.908853091009225 ; 1.909101557924329
>>>>>
>>>>> The distance (along the highway) from the turnoffs
>>>>> to Undip / Lansford airstrips is 8.1km in JOSM. So
>>>>> the latlon calculation is correct. However, the
>>>>> dimension calculated from the pixels isn't.
>>>>> _______________________________________________
>>>>> dev mailing list
>>>>> dev at openstreetmap.org <mailto:
>>>>> dev at openstreetmap.org>
>>>>> https://lists.openstreetmap.org/listinfo/dev
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> dev mailing list
>>>>> dev at openstreetmap.org <mailto:dev at openstreetmap.org>
>>>>> https://lists.openstreetmap.org/listinfo/dev
>>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> _______________________________________________
>>>> dev mailing list
>>>> dev at openstreetmap.org
>>>> https://lists.openstreetmap.org/listinfo/dev
>>>>
>>>>
>>>
>>>
>>
>>
>
> --
> Tom Hughes (tom at compton.nu)
> http://compton.nu/
>
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